uniform sampler2D source;

 // vertical blur 
const float blurSize = 1.0/512.0;
 
void main(void)
{
   vec4 sum = vec4(0.0);
 
   // blur in y (vertical)
   // take nine samples, with the distance blurSize between them
   sum += texture2D(source, vec2(t, s - 4.0*blurSize)) * 0.05;
   sum += texture2D(source, vec2(t, s - 3.0*blurSize)) * 0.09;
   sum += texture2D(source, vec2(t, s - 2.0*blurSize)) * 0.12;
   sum += texture2D(source, vec2(t, s - blurSize)) * 0.15;
   sum += texture2D(source, vec2(t, s)) * 0.16;
   sum += texture2D(source, vec2(t, s + blurSize)) * 0.15;
   sum += texture2D(source, vec2(t, s + 2.0*blurSize)) * 0.12;
   sum += texture2D(source, vec2(t, s + 3.0*blurSize)) * 0.09;
   sum += texture2D(source, vec2(t, s + 4.0*blurSize)) * 0.05;
 
   gl_FragColor = sum;
}